Lightly spray a 9x13-inch pan with cooking spray. Set aside.

In a large skillet over medium heat, cook the sausage until no pink remains. Drain all the grease except for 1 tablespoon.

Place the onions and bell peppers in the same skillet and cook briefly just to soften, about 2-3 minutes. Remove from heat.

In a large bowl, whisk the eggs until well beaten. Pour in the milk, heavy cream, ground mustard, salt and pepper. Whisk to combine.

Arrange the croutons in the bottom of the baking dish in a single layer. Pour the sausage, peppers and onions over the croutons. Then sprinkle on the cheese.

Carefully pour in the egg mixture and cover with plastic wrap. Refrigerate for at least an hour or up to overnight.

When ready to bake, preheat the oven to 350°F. Bake for 50-65 minutes or until the center is set and doesn’t jiggle. A knife inserted in the center should come out clean and no liquid egg mixture should run out.

Let the casserole cool for 10-15 minutes before serving.

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